Thorium Nuclear Reactors

Study Notes

Thorium is three times more abundant in nature than uranium and occurs mainly as the 'fertile' 232Th isotope.
232Th can breed human-made 'fissile' isotope 233U efficiently in a thermal neutron reactor.
The half-life of 232U is only 73.6 years, and the daughter products have very short half-lives, making it a potential intrinsic proliferation-resistant barrier.

Several experimental and prototype power reactors were successfully operated from the mid-1950s to the mid-1970s using (Th, U)O2 and (Th, U)C2 fuels in high-temperature gas-cooled reactors (HTGR), (Th, U)O2 fuel in light water reactors (LWR), and Li7F/BeF2/ThF4/UF4 fuel in molten salt breeder reactors (MSBR).
232Th and 233U are the best 'fertile' and 'fissile' materials, respectively, for thermal neutron reactors, and 'thermal breeding' has been demonstrated for (Th, U)O2 fuel in the Shippingport light water breeder reactor (LWBR).

For incineration of weapons-grade plutonium (W-Pu) or civilian Pu in fast reactors utilizing a 'once-through' cycle, (Th, Pu)O2 fuel is a better option compared to (U, Pu)O2.
In thoria matrix fuel, plutonium is not bred, instead, 232U is formed in the spent fuel, ensuring proliferation-resistance due to the high gamma radiation associated with the daughter products.

AHWR 300 is a 300 MW(e) reactor designed to maximize the energy potential of vast thorium resources (~518 000 tonnes - in terms of thorium metal).
Salient features of AHWR 300 include:
- Use of ThO2-based driver fuel: Zircaloy 2 clad (Th, Pu)O2 and (Th, 233U)O2 fuel pin clusters
- Heavy water moderator as heat sink
- Boiling light water coolant
- Vertical pressure tube
- Heat removal through natural circulation
- On-power fuelling

The high thermal neutron absorption cross-section of 232Th (7.4 barns) compared to that of 238U (2.7 barns) reduces the fraction of thermal neutron absorption in coolant, moderator, and structural materials, enabling the use of light water as coolant while retaining heavy water as moderator for better neutron economics.
Heat removal in boiling mode is preferred for overall reduction of coolant quantity and improvement in steam cycle efficiency.

The possibility of registering a positive void coefficient of reactivity with light water coolant has been countered by reducing the lattice pitch, making the lattice under-moderated, and using a burnable absorber (Dy2O3) in the fuel cluster.
The reactor has been engineered to obtain a negative void reactivity and, in turn, inherent safety.

The AHWR core will be replicated by replacing the 9 metallic natural uranium clusters in a 3x3 array in the central region of the reference core by AHWR fuel clusters at a square pitch of 250 mm.
Two types of fuel clusters will be used in sequence in the core of the ZPCF:
- Set 1: 9 numbers of 54 fuel pin clusters – all pins made up of (Th, Pu)O2 fuel
- Set 2: 9 numbers of 54 fuel pin clusters containing 24 pins of (Th, Pu)O2 and 30 pins of (Th, 233U)O2

The ACR is designed to be an economical reactor choice for today with enhanced safety and reliability, while meeting expectations for sustainability.
The ACR reference design provides a design life of 40 years with an option to extend to 60 years.
The ACR is a heavy water moderated, light water cooled reactor.
Evolutionary changes from the CANDU 6 allow for a compact reactor core design, with the core of the 700 MW class having 284 channels, with 12 CANFLEX fuel bundles per channel.

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